1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular the present invention is directed to a fiber optic cable having improved flame and smoke retardant characteristics.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today""s technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to produce optical fibers of better quality at faster rates and lower costs, while providing adequate safety protections.
Many of the areas of use for optical fibers, such as communications, require the optical fibers be protected from various destructive elements, such as adverse weather, moisture, impact damage, and fire. When fiber optic cables are used in commercial and residential buildings they must meet certain flame, fire and smoke retardation requirements. For example, one of the most stringent tests requires placing a fiber optic cable horizontally over an open flame for 30 to 40 minutes, and to pass the test the cable can not generate an excessive amount of dense smoke.
Many components in fiber optic cables, such as buffer tubes and central strength members, generate excessive smoke when heated to high temperatures. In existing cables when a heat source is applied the heat propagates radially towards the center of the cable, where all of the smoke generating components are located. This is mainly due to the materials used, which do not allow heat to propagate along the length of cable easily.
There have been advances in cable fire and smoke retardation for fiber optic cables, however, many of the existing advances and cable configurations still fail the above mentioned test. Therefore, there is a need for a cable structure which does not generate dense smoke within the 30 to 40 minute window.
The present invention is directed to a fiber optic cable structure having improved flame and smoke retardant properties. This is accomplished by adding two layers of Mica tape, or similar material, around the cable core (which includes mainly the buffer tubes and, central strength member), where between the two layers of Mica tape is a layer of flame-retardant yarn which forms air gaps between the layers of tape. The first layer of Mica (outermost) acts as a fire (hot plasma) barrier, while the air gaps resist heat propagation from the outer Mica layer to the inner Mica layer and the cable core components. The two tape layers also allow the heat from an applied flame to propagate along the length of the cable instead of having the heat radiate inward from the point where the flame is applied. By allowing the heat to propagate longitudinally, the heat is dissipated over a larger area and, as such, passes less heat to any one point in the center core of the cable. This greatly lengthens the time needed to cause the center core components to start generating significant amounts of dense smoke. Further, even when smoke is generated, the tape layers act as a smoke barrier, preventing the smoke from escaping from the center core.
In an alternate embodiment, a powder having low-thermal conductivity is placed within the air gaps, between the two tape layers, to aid in preventing heat propagation from the outer layer to the internal components. In another embodiment, a limestone or similar powder with low thermal conductivity is placed in the cable to be decomposed under high heat and to generate carbon dioxide as a means to absorb heat and aid in extinguishing or inhibiting fire. Further, as an alternative to the Mica tape, aluminum (or any other thermally conductive metal) foil can be used to rapidly conduct the heat from the flame longitudinally along the cable, which reduces the propagation of the heat in the radial direction (i.e. towards the center of the cable).
In the present invention the tape material and thickness and the air gap size are optimized to reach the best performance in smoke and fire retardation.